A wind turbine is a device that utilizes the wind to power a machine. If the turbine is connected directly to the machine, such as a pump or a millstone, the turbine is usually referred to as a windmill. If the turbine is connected to a generator which, in turn, supplies a machine with electrical power the turbine is often referred to as a wind generator.
The most common wind turbine is one that rotates about a horizontal axis. Typical turbines of this type have a rotor shaft and an electrical generator positioned at the top of a tower and pointed into the wind. Small turbines are pointed by a wind vane whereas larger turbines generally use a wind sensor coupled with a motor.
The speed at which a wind turbine rotates must be controlled. Most importantly, speed control permits the aerodynamic efficiency of the turbine to be optomized for use in light winds. Also, speed control keeps a generator within its torque limits. Of equal importance, speed control allows a wind turbine to be slowed in high winds to prevent it from exceeding its own structural limits and those of its supporting tower. Finally, limiting the rotational speed of a turbine reduces the noise that it emits.
Speed control over a wind turbine is normally exerted by aerodynamic stalling and furling as well as electrical and mechanical braking. Stalling and furling, the preferred methods of slowing wind turbines, work by swiveling the turbine's blades so that either a flat side or an edge of the blades face into the wind. Electrical braking, however, requires the dumping of energy from a generator into a resistor bank, converting the kinetic energy of the turbine's rotation into heat and causing the turbine to slow down. Mechanical, drum, or disk brakes are often used to hold the turbine motionless for repair. Mechanical brakes are usually applied only after blade furling and electromagnetic braking have reduced turbine speed since mechanical brakes would wear quickly under typical loads. One major problem in designing wind turbines is getting them to slow quickly enough should a gust of wind cause sudden acceleration to unacceptable rotational speeds.